Light dimming is performed by adjusting voltage through a lamp and has been possible for many decades by using adjustable power resistors and adjustable transformers. The problems of such light controlling methods have been that they are big, expensive, inefficient and are difficult to control from a remote location.
Recent conventional dimmers generally work by varying the “duty cycle” of an AC voltage that is applied to the lights being controlled. Between 1960 and 1970, a TRIode for Alternating Current (TRIAC) became available in the market. Using a TRIAC component allowed dimmers to be made small and inexpensive. Currently dimmers with a TRIAC component are used in many locations such as homes, restaurants, conference rooms and stage lighting.
The TRIAC component, when triggered by a sufficient voltage, will conduct current as long as current through it is not zero. In an AC circuit the TRIAC is usually triggered twice for every AC voltage cycle. Thus, the TRIAC component is useful as a switching element for all types of AC electronic applications. The TRIAC component triggers at a position along the AC sinusoidal voltage which chops the AC voltage between the zero voltage and the trigger position. For example, if the AC voltage applied to a light bulb, is chopped for half of each AC voltage cycle, the light bulb will appear to be much less bright than when the full AC voltage cycle is applied. Moreover, the conventional dimmer uses a brightness knob to set the trigger position in each AC voltage cycle to switch the light on.
The foregoing dimmer with the TRIAC component works well for incandescent lights but is not suited for lights that require ballasts. Using the conventional TRIAC dimmer on a gas discharge lamp, cold cathode fluorescent lamp (CCFL), carbon nanotube lamp (CNL) or white LED (WLED) lamp without a specially designed electronic ballast may result in improper lamp operation or damage of the lamp.
Another dimming method used in a gas discharge lamp is a pulse width modulation (PWM) dimmer. The PWM dimmer of the gas discharge lamp turns the lamp on and off at a fixed frequency but at a varying duty cycle. The fixed frequency is chosen higher than a human eye can detect. The duty cycle determines the brightness of the lamp. The PWM dimmer produces efficient operation at any level of brightness.
However, integrating a TRIAC dimmer with the PWM dimmer is complicated since a truncated voltage produced by a TRIAC is abbreviated not only in time but also in voltage. Some conventional ballasts work well with constant AC voltage. The voltage required for a ballast to operate properly may be high when compared to the voltage of the truncated voltage available from the TRIAC and as such may not be able to turn on the ballast.
Moreover, if a conventional passive fluorescent ballast receives the truncated voltage instead of a smooth AC voltage, a transient voltage of the truncated voltage might feed into some magnetic element and cause immediate lamp failure.
Most ballasts first rectify the incoming AC voltage to provide a quasi DC voltage that drives the lamp. Some lamp designers use a capacitor to store power to keep the ballast “alive” during the OFF period of the TRIAC. However in this case, the dimming information in the truncated voltage from the TRIAC would no longer be available because it has been filtered out, preventing it from being utilized for PWM dimming. Furthermore, the capacitor needed for storing the power may be expensive and large.
Therefore, lighting manufacturers are eager to develop a pulse-width modulation control device with a TRIAC dimmer that reliably dims all ballasted lighting using TRIAC dimmers with efficient PWM dimming features.